A zoo, each being a different display. Each exhibit is a 'rare' or fanciful creation, think Simulacrums. Each 'subject' responds to life energy, coming 'alive' only when someone is with them. Pulled this in a 'prison' for high end BBEGs captured by a LE cult with a variation of Soul Jar. The exhibits do not know they are artificial and unable to advance in levels, thinking some agency is crimping their abilities.
The PCs had freed several before they identified them and paused the revival of more. Currently they are hunting five of the escapees, all level 7 to 9 casters of ill temper.
One captured the party and bargained their release: the Paladin swearing oaths, to help her back to her right self. In the rarely run game, I converted her into a Witch and am looking for a better archetype. She escaped bereft of spells, but was loaded day three when she suckered them. She needs some real social skills as all her minions died out hundreds of years back. And what would be a good bloodline...
A plane with all male of one species, and all females of another, to test cross-species breeding and trait dominance. You could do several of these.
A plane where a new predator is introduced every century or so to test their competitiveness.
A zoo is a good idea, with themes. An aviary plane (all birds). Freshwater and saltwater aquarium planes. A plane of "extinct" animals. Several of these.
A prison. A variation on the zoo. Maybe to examine the differences in human behavior. So a plane of LE people, a plane of CG people, etc. He watches the different social dynamics that develop.
Maybe look at the different Vaults from Fallout. Each was supposedly a large fallout shelter to protect a sizable number of people from nuclear war, but in reality were used to run psychological experiments involving drugs, violence, conditioning, eugenics, etc.
"Always play with there minds " - Lucus, Empire Records
A labyrinth with a constant fog (5' visibility), then have the walls move...
A plane consisting entirely of intellectuals and artists: wizards, philosophers, poets, musicians, etc. Just a paradise of learning and culture. The plane is mildly positive energy-aligned and timeless, keeping all beings therein eternally youthful and precluding the need for resources (e.g., food is only taken for pleasure, never for sustenance).
Valhalla. Carouse in a mead hall, then go kill some (<=4HD) giants.
The ultimate sensory deprivation chamber. No gravity (so no interoception); no medium capable of transmitting sound or visual phenomena.
Planes dedicated to the pure, distilled essence of single experiences. For example, a plane of reticulate twisting hallways where one hears whispers of ultimate knowledge, always hiding just behind the next corner; a plane where one is continuously born; a plane where every sentient creature continuously falls in love with whomever it sees, weeping in perpetual joy.